Electronic Devices

Classification of Metals, Conductors and Semiconductors

  • The band formed by a series of energy levels containing the valence electrons is known as VALENCE BAND.
  • Valence Band may be partially or completely filled with electrons depending on the nature of crystal.
  • Valence Bond is the highest filled energy band.
  • The band formed by a series of energy levels containing the conduction electrons is known as CONDUCTION BAND.
  • Conduction Band may be empty or partially filled with electrons.
  • Conduction Band is the lowest unfilled energy band.
  • A part of the energy band which is not occupied by any electron is known as FORBIDDEN BAND.
  • The energy gap between the valence band and the conduction band is called FORBIDDEN ENERGY GAP.
  • No electron will exist with an energy level in forbidden gap.
  • The substances having high electrical conductivity are known as CONDUCTORS.
  • The valence band and conduction band overlap. En! metals
  • Those substances having poor electrical conductivity are INSULATORS. The valence band and conduction band are separated by large forbidden gap (≈ 5 – 9eV) Ex: Diamond
  • The substances whose electrical conductivity lies between conductors and insulators are called SEMI CONDUCTORS.
  • The forbidden energy gap between conduction and valence band is very small (≈ 1 eV) Ex: Si & Ge
  • For silicon forbidden energy gap is 1.1 eV and for germanium 0.72 eV.
    Eg ≈ KT
  • For diamond, forbidden energy gap is 6eV.
    Eg >> KT
  • At O Kelvin, pure semiconductor behaves as a perfect insulator.
  • On adding suitable impurities to the semiconductor, their conductivity increases by large amounts.
  • The charge carriers which are responsible for conduction in semiconductors are free electrons and holes.

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For extrinsic or doped semiconductor
n_{e} \cdot n_{h} = n_{i}^{2} \Rightarrow n_{e} = \frac{n_{i}^{2}}{n_{h}}